In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Modern high productivity chip forming machining of metals requires reliable cutting tool inserts which posses high wear resistance, good toughness properties, ability to produce work pieces with high surface finish, resistance to chip hammering, be able to produce reasonable low cutting forces and have excellent resistance to plastic deformation.
Modern cemented carbide tools are generally in the shape of an indexable insert clamped in a tool holder, but can also be in the form of a solid carbide drill or a milling cutter. Cemented carbide cutting tool inserts coated with various types of hard layers like TiC, TiCxNy, TiN, TiCxNyOz and Al2O3 have been commercially available for many years. Several hard layers in a multilayer structure generally build up such coatings. The sequence and the thickness of the individual layers are carefully chosen to suit different cutting application areas and work-piece materials.
The coatings are most frequently deposited by Chemical Vapour Deposition (CVD) or Physical Vapour Deposition (PVD) techniques.
The CVD-technique has several advantages. It allows large coating batches, produces coatings with good coating thickness distribution on complex shaped inserts, can be used to deposit electrical non-conducting layers like Al2O3 and ZrO2. Many different materials can be deposited in the same coating run, e.g. Al2O3, TiC, Ti(CxNy), TiN, Ti(CxNyOz), Zr(CxNy), Ti(CxNy) and ZrO2.
The CVD technique is generally conducted at a rather high temperature range, 900-1050° C. Due to the high deposition temperature and due to a mismatch in thermal coefficient of expansion between the deposited coating materials and the cemented carbide tool substrate, CVD produces coatings with cooling cracks and tensile stresses. CVD coated inserts therefore appear more brittle than PVD coated inserts when used in a cutting operation.
PVD processes run at a significantly lower temperature, 450-650° C. and are performed under strong ion bombardment which leads to crack free layers with high compressive stresses. The high compressive stresses and the absence of cooling cracks make PVD coated tool inserts much tougher than CVD-coated tool inserts and are therefore often preferred in interrupted cutting operations, e.g. milling. The drawback of PVD coated inserts are generally the lower wear resistance and insufficient coating adhesion when compared to CVD coated inserts.
Hence, there is constantly a strong strive to find means to improve the toughness behaviour of CVD-coated tool inserts while keeping the high wear resistance.
Post treatment of coated cutting inserts by brushing or by wet blasting is disclosed in several patents. The purpose is to achieve a smooth cutting edge and/or to expose the Al2O3 along the edge line as disclosed in U.S. Pat. No. 5,851,687 and in EP 603 144 or to obtain the Al2O3 as the top layer also on the rake face in those cases when TiN is used as a wear detection layer at the flank face as disclosed in U.S. Pat. No. 5,861,210. Every treatment technique that exposes a CVD coating surface to an impact force such as wet- or dry blasting will lower the tensile stresses of the coating and thereby improve the toughness of the coated tool as disclosed in US 2006/0204757A1. A dry blasting method is disclosed in EP 1 311 712. Here a very high blasting pressure is used to obtain high compressive stresses. Such high blasting pressure will deteriorate the coating and produce an uneven edge line, this is particularly pronounced for inserts with sharp edges (radius <35 μm).